The use of energy delivered by optical fibers in medical procedures and industrial processes is known. The desired effects caused by the energy can be thermal, photodisruptive or photo-chemical.
In many medical procedures, energy delivered by optical fibers is used for its thermal effect. That thermal effect is dependent upon the specific way the energy delivered by the optical fiber is transferred to thermal energy in the tissue. One laser used in medical procedures is the carbon dioxide (CO.sub.2) laser whose wavelength at 10.6 .mu.m makes it opaque to cellular water. The CO.sub.2 laser is therefore totally absorbed by water and rapidly converted to thermal energy over a very short distance (&lt;100 .mu.m).
Recently, Nd:YAG laser systems, coupled to silica fibers with either sculptured or bare tips or with sculptured sapphire tips, have shown great benefits as surgical tools when used for certain procedures. Using these laser systems with a bare fiber, photocoagulation to tissue depths of 4 to 5 mm in tissue can be attained in a non-contact mode. In a contact mode, incision and cauterization of the nearby tissue can be achieved.
These two capabilities, though providing the surgeon with new and powerful tools in performing procedures that are very close to being hemostatic, have as yet to be integrated into a full spectrum of optical fiber surgical systems. At one extreme, only photocoagulation can be achieved in a non-contact mode while at the other extreme only incision can be achieved in a contact mode. Between these two extremes, there is a range of combined and controlled photocoagulation and incision that would be highly desirable, and a fiber optic system that could provide this full-spectrum capability would provide the surgeon with a broad range of new surgical capabilities to meet the specific needs of a broad range of surgical procedures.
The most common optical fiber material used for the delivery of energy at the present time is silica glass. Indeed, the same glass chemistry presently used in typical optical fibers for laser surgery is also used in telecommunication optical fibers. These optical fibers are capable of transmitting light energy through very small diameters and they can be threaded to almost any part of the body creating little or no damage to surrounding normal tissue. As a result, fiber optic delivery systems are useful in conjunction with endoscopic procedures and catheter-based delivery systems.
Common silica fibers can be highly effective in photocoagulating tissue in a non-contact mode. However, when common silica fibers come into contact with tissue or blood there is significant thermal-mechanical damage to the fiber and disruption of energy transmission. Because of the absorption of light energy at the tissue contact surface, the fiber tip is rapidly heated to high temperatures thereby destroying the tip. The effect on the surrounding tissue is variable and has an unpredictable tissue damage pattern.
The thermal-mechanical breakdown of the optical fiber that follows the use of the fiber in a contact mode also typically result in contamination of the incisional site with silica glass fragments in addition to preventing further photocoagulation due to absorption of energy by the degraded tip. These fragments may present a bio-hazard as their effect on tissue has not been fully studied. Perhaps more importantly, the use of a technique for precise laparoscopic dissection that creates a variable tissue effect with significant lateral coagulation is less than optimal.
Attempts have also been made to provide optical fibers which, for example, have sapphire contact tips or infrared absorbing materials at their tips to control the dispersion of energy from the tip and/or the thermal-mechanical breakdown. These attempts have not met with success because, even though they may be specifically designed for contact applications, all of the fibers suffer from the same thermal and/or chemical degradation described above for the silica fiber tips.
As a result, although known optical fibers can provide adequate performance when used in non-contact applications to accomplish photocoagulation or other photo-chemical effects, the reality facing surgeons is that contact between the fiber tips and tissue is extremely difficult to avoid due to the close quarters in which these devices operate. After the initial contact and degradation occurs, the performance of the fiber can no longer be accurately predicted. If predictable characteristics are required, a new fiber tip must be created, either by cutting and polishing a new tip or by replacing the entire fiber. Both of these options increase the cost of the procedure and increase the time need to complete it.
One approach to address these problems was developed by one of the inventors of the present application and described in U.S. patent application Ser. No. 08/209,002, filed on 10 Mar. 1994, which is hereby incorporated by reference, describes the use of an optical fiber having a core of silica glass doped with titanium dioxide. That particular material provides an extremely low coefficient of thermal expansion. As a result, optical fibers made with sufficient amounts of titania (typically 6-8%) do not suffer from the thermal expansion degradation effects described above for fibers constructed of other materials.
Those fibers are, however, difficult to produce in that the preforms should be pulled into fibers in an air or oxygen atmosphere to maintain the titanium in the doped preform in its highest (Ti.sup.+4) oxidation state. If an air or oxygen atmosphere is not used, then the resulting fibers may contain significant amounts of reduced titanium (Ti.sup.+3) which absorbs energy in the visible and near infrared regions. As a result, those fibers suffer from a loss in performance when used with the significant numbers of medical lasers operating in the visible/near infrared regions.
Another disadvantage of optical fibers with silica-titania cores is that, although they transmit significant amounts of energy, they are not as efficient as typical silica glass cored fibers. In general, silica-titania glass cored fibers transmit only about t 75-80% of energy in part because the commercially available preforms are not designed for use in fiber optics. In contrast, the rate of transmission of typical silica glass cored fibers is at least about 95%.
Optical fiber systems currently used for medical procedures, silica or otherwise, remain useful as photocoagulating or incisional tools only so long as they are used in non-contact procedures. Given the close nature of the environments in which these devices operate, however, contact and the resulting degradation are difficult, if not impossible, to avoid. Furthermore, the use of silica-titania glass fibers addresses the thermal degradation problems, but suffers from other disadvantages.
In addition to the methods of medical treatment useful using optical fibers, many industrial processes relying on thermal effects produced by laser energy delivered by optical fibers for cutting, brazing, welding, soldering, ablation, marking and other industrial processes. Many of those processes expose the optical fiber tip to thermal energy that results in thermal/chemical degradation of the tip similar to that found in the medical procedures described above. In some instances it may be desirable to place the fiber tip in contact with the materials being processed, but that option may not be available because of the damage it would cause to the fiber tip. The problems of thermal degradation may, in fact, be more acute in industrial processes due to the typically higher energy levels associated with industrial lasers.
One attempt at preventing or controlling degradation of optical fiber tips in industrial processes including shrouding them to provide a positive pressure environment to prevent contact between the tips and the materials being heated. Such approaches, however, increase the cost and complexity of systems incorporating optical fiber delivery systems.
As a result, there is a need for an optical fiber that provides predictable operating characteristics in thermal either a contact or non-contact mode, can transmit significant amounts of energy, and can be manufactured using existing fiber technology.